Polyorganosilsesquioxanes with a regular ladder structure, due to their excellent plasma-resistant and heat-resistant properties, have conventionally been considered to be useful as, for example, resist materials and heat-resistant coating properties. Of these polyorganosilsesquioxanes having a ladder structure, those having aryl or alkyl groups are synthesized according to various methods. In one such method, a polyorganosilsesquioxane compound with for example aryl groups such as phenyl or toryl, or higher alkyl groups such as isobutyl or isoamyl, and which is soluble in organic solvents, is synthesized by hydrolysis and condensation polymerization of an organotrichlorosilane with said organic groups in an organic solvent at a low temperature (J. Amer. Chem. Soc., Vol. 82, P 6194 (1960), J. Polymer Sci. Vol C-1 P. 83 (1963)). In another method, a soluble polyorganosilsesquioxane with C.sub.1 -C.sub.3 lower alkyl groups is synthesized by hydrolysis and condensation polymerization of an organotrichlorosilane at the interface between an aqueous layer and an organic layer (Unexamined Japanese Patent Application (Japanese Patent Kokai) No. 50-111198).
Also, Japanese Patent Kokai No. 62-16212 discloses the synthesis of a high molecular weight polyorganosilsesquioxane with lower alkyl groups by hydrolysis of an organotrichlorosilane at the interface between an aqueous layer and an organic layer followed by condensation polymerization under an inert gas at high pressure.
In this case, the reactivity of the organotrichlorosilane in the hydrolysis reaction is very high. The reaction may therefore be controlled by carrying out the above hydrolysis either at low temperature and/or at the interface between the two layers, and cage-type prepolymers such as T.sub.6, T.sub.8 and T.sub.10 having regular structures with considerable distortion are synthesized. These cage-type prepolymers are then heated to induce condensation polymerization, while maintaining regular structures and synthesize polyorganosilsesquioxanes with a regular ladder type structure.
In the above method, however, because the hydrolysis of the above organotrichlorosilane has to be carried out under controlled conditions at low temperature, there is a risk of overrun and the method is not suited to mass production.
The present inventors carried out intensive studies to overcome these problems and found that the desired polyorganosilsesquioxanes with a regular ladder structure could be easily mass produced from prepolymers synthesized by a heat-induced condensation polymerization of partially alkoxylated triacetoxysilanes in the presence of sodium hydrogencarbonate, and then carrying out a condensation polymerization of the prepolymers obtained in the presence of catalysts.